1. Field of the Invention
The present invention relates to an ischemia detector of the type having a workload sensor for sensing the workload of a patient. The invention also relates to a heart stimulator including such a detector and a method of detecting ischemia.
2. Description of the Prior Art
Cardiac ischemia is a condition related to lack of blood flow and oxygen to the heart muscle. Such a condition arises when a coronary artery is narrowed or occluded for a short time such that flow of oxygen-rich blood to the heart is reduced or prevented. If the ischemia is severe or lasts for too long time, it can cause a heart attack (myocardial infarction) and can result in heart tissue death.
A temporary blood shortage to the heart causes, in most cases, pain or angina pectoris, but in some cases the patient feels nothing. The latter case is called silent ischemia.
Angina usually occurs when the heart's need for blood exceeds the supply of blood. For example, running to catch a bus could trigger an attack of angina while walking might not. An angina might occur during exercise, strong emotions or extreme temperatures. Persons having a coronary artery spasm may have angina even when resting.
For persons suffering from an unstable angina, the chest pain is unexpected and usually occurs while the persons are resting. Inflammation, infection and secondary causes can also give rise to unstable angina. The cause of a form of unstable angina called variant or Printzmetal's angina is coronary artery spasm. Unstable angina is an acute coronary syndrome and should be treated as an emergency.
Persons having angina in addition may have also undiagnosed episodes of silent ischemia. Silent ischemia may also cause disturbances in the heart rhythm. Abnormal rhythms, like ventricular tachycardia or ventricular fibrillation, can interfere with the heart's pumping ability and cause fainting or even sudden cardiac death. A silent ischemia can lead to a heart attack without any prior warning. Detection of silent ischemia is consequently very important. Heart muscle disease (cardiomyopathy) caused by silent ischemia is among the more common causes of heart failure.
Silent ischemia is very common. The American Heart Association estimates, for instance, that 3 to 4 million Americans have episodes of silent ischemia. Especially persons who have had previous heart attacks and those who have diabetes are in the danger zone for developing silent ischemia.
Angina is a signal from the heart muscle of insufficient oxygen supply to the heart tissue due to diminished blood supply. A heart attack is the most extreme state of oxygen deprivation, in which whole regions of heart muscle cells begin to die because of lack of oxygen. The ejection fraction is often very low in the acute stage.
Even a heart attack may not be unbearably painful at first, permitting its victim to delay seeking treatment for as much as 4 to 6 hours after onset of the attack. By then the heart may have suffered irreversible injuries.
The longest running heart study, the Framingham Heart Study in United States, indicates that about one heart attack of four produces no symptoms, or at least no symptoms which the victim associates with a heart problem.
So-called silent heart attacks are only the most extreme case of the still more prevalent condition silent ischemia described above. The prevalence of silent heart attacks is high for elderly and diabetic patients.
U.S. Pat. No. 6,016,443 describes an ischemia detector including a detecting unit which identifies a state of ischemia as existing upon the occurrence of a predetermined relation between sensed repolarization and sensed workload of the patient.
U.S. Pat. No. 6,233,486 discloses an ischemia detector, wherein an ischemia is detected from an established relation between the systolic pressure of a subject and the subject's heart rate. It is mentioned that the described technique is useful for detecting also so-called silent ischemia.
In U.S. Pat. No. 6,256,538 an implantable heart stimulator with an ischemia detector is described, wherein the stimulation rate is reduced in response to the detection of ischemia. It is suggested to detect ischemia in one of the following ways, by analysis of recorded IEGMs or ECGs, by analysis of ST segments and T-waves, by analyzing measured AC impedance in a ventricle, by measuring sound absorption in heart tissue, by comparing measured differences between systolic and diastolic pressures from consecutive heartbeats, or by measuring cardiac output. The possibility to detect silent ischemia with these techniques is also mentioned.
U.S. Pat. No. 6,264,606 discloses an ischemia detector wherein an ischemia is identified upon the occurrence of a predetermined relation between sensed workload and sensed breathing activity, said predetermined relation meaning a sensed low workload and a simultaneously sensed high breathing activity.